1. Field of the Invention
This invention relates to oscillators of the bridge type and more particularly relates to the use of such oscillators as part of apparatus for detecting metal particles, detecting the proximity of objects, detecting temperature changes and the like.
The specific embodiment of the invention discloses an apparatus for sensing undesired metal particles which frequently become included in products during the manufacturing process. Such particles are referred to as tramp metal particles or simply tramp metal, and apparatus for detecting the presence of such particles are frequently called tramp metal detectors. These detectors usually include an oscillator circuit.
2. Description of the Prior Art
Many oscillators employing positive and negative feedback in an amplifier circuit are known in the art including oscillators that employ a bridge circuit as part of the feedback network. Examples of such oscillators are the Meacham bridge type described in U.S. Pat. No. 2,163,403 and the Wien bridge type as described for example in "Vacuum Tube Oscillators" by William A. Edson at pp. 138-142, John Wiley & Sons, Inc., N.Y. 1953. Such oscillators have been employed as part of a proximity detection apparatus as is described for example in U.S. Pat. No. 3,870,948. When used in such apparatus, the oscillators of the prior art necessitate the use of complex and expensive techniques to ensure stable detection sensitivity over variations in environmental conditions such as temperature, humidity, vibration, etc. which are not required by the oscillator of the present invention.
A common tramp metal detection apparatus is a Balanced Coils Metal Detector that consists of a center drive coil connected to an oscillator circuit and two search coils, one placed on each side of the drive coil. The search coils are carefully balanced and connected in opposition so as to obtain zero output signal. In order for a balanced coils metal detector to achieve a satisfactory sensitivity continuous balancing of the search coils utilizing complex added circuitry and frequent manual resetting of balance controls is required. Maintaining adequate and stable sensitivity of this circuit in the typical industrial plant environment requires almost continuous operator attention.
An additional disadvantage of the balanced coils metal detector is the fact that in practical applications it is limited to a geometry which produces a magnetic search field in a single direction. Problems of mutual interaction and the resulting complexity of stabilization circuitry makes multiaxial systems based on the balanced coil detector impractical.
Another prior art metal detector known as a Tuned Search Coil Detector uses a single search coil as the tank coil of a conventional oscillator circuit. The operation of this detector is based on the fact that the Q of the coil is changed by the presence of a metallic object. The change in Q changes the output amplitude of the resonance circuit oscillator and is utilized to indicate the presence of a metallic object. Metal detectors based on the Tuned Search Coil principle also require manual adjustment of the oscillator tank circuit and have a sensitivity which is even less than that achieved by Balanced Coils Metal Detectors.